Skewed Vertical Arrays

Not long after my article on arbitrarily spaced driven verticals a reader posed an interesting question, one that was only briefly touched on in the article: can the array be steered by adjusting the element phases? The actual question wasn't exactly that so I am paraphrasing based on our exchange. Unfortunately the answer is no, not really.

With 2 elements in a driven array you can steer pattern nulls but not the main lobes. To steer a null you adjust the phasing so that at a selected direction (azimuth and elevation) the phase difference is 180°. In the general case there will be more than one null, but no more than two for the optimum spacing of ¼λ. This can be useful for attenuating local noise sources, but it is not useful for a transmit antenna.

With 3 or more elements there are improved steering options. The design can be complex since power splitting and phasing must be adjusted for all the elements, and the vertical elements cannot be collinear (in a line). Examples include triangle arrays and 4-squares. General steering isn't worthwhile since the patterns are broad enough that several fixed, switchable directions are sufficient.

Since there are ample designs and commercial product for 4-squares and other multi-element driven vertical arrays, I will focus this article on steering of 2-element end-fire driven arrays and 3-element reversible vertical yagis. 

The steering will be done by skewing the elements hung from a common catenary (rope) between non-conductive or non-resonant supports. The supports are thus excluded from the models. Skewing can be useful when the positions of the supports do not allow the catenary to be run in the desired direction. That is, we have a means to put arbitrarily positioned supports to good use.

I previously showed how to skew a horizontal wire yagi for the same catenary challenge. The design ideas in this article can be seen as complementary to that one. The examples are for 160 meters since the low bands are where these designs are likely to be attempted. It is easy to scale these antennas to other bands.

Modest skewing

I adapted this antenna from a 2-element 160 meter end-fire with optimal ¼λ element spacing and 90° phase shift. The base of each element has an orthogonal offset of 10 meters so that the angle to the catenary of the line connecting the element bases is about 26°.

For convenience, the model was done with perfect ground. This is acceptable since the pattern skew is not affected by ground quality. For a real antenna the elevation angle for optimum F/B is dependent on ground quality and element phasing.

The skew of the azimuth pattern is 20°. This is close to the value of the physical skew of the element bases. That's a good result. The bases are separated by 0.28λ because of how the skew was achieved. Despite that change the phase shift to preserve the F/B performance remains at 90°. As mentioned above, for real ground the phase shift should be adjusted to optimize F/B at a more realistic elevation angle.

Greater skewing

As you increase the skew angle the array performance deteriorates. Even so, you can push quite far and get a good result. In the example below the bases of the 2-element end-fire are skewed 45°. Unlike the previous example, the bases are rotated on a virtual circle so that they retain their ¼λ separation. The apex of the elements is lower due to the tilt.

Pattern skew is, again, almost the same as the physical skew: 40°. The phase shift was increased to 130° to get the best F/B. F/B is better with a smaller skew. 

I included the elevation plot since there is a significant change. The horizontally polarized radiation due to the greater element tilt fills the pattern at high angles. Depending on your operating interests this can be good or bad. For a DXer the high angle radiation is not useful in most cases. For a contester it can be beneficial to work closer stations.

Vertical yagi skew

For this antenna I took an existing 160 meter model with 3 wire elements and shifted the base positions of the director and reflector by 15 meters orthogonal to the catenary. Element spacing is equal (30 meters) since the antenna was designed to be reversible. As with the first end-fire the angle to the catenary of the line connecting the element bases is 26°.

Pattern skew of 20° is only slightly less than the base skew. That's better than what I achieved with the skewed horizontal yagi referenced earlier. It is interesting that the pattern skew is the same for the 2-element end-fire that also has a 26° physical skew.

Adjustment of the elements to optimize for the new configuration is recommended to get the best from this antenna. I skipped that step since it does not affect the skew angle.

Geometry of skewing

The reason why a skewed vertical array works is that the field strength of an antenna (or antenna element) is in proportion to the current. Since the current on a ¼λ vertical is greatest at the base and the height of the average current is low, the virtual position of the vertical is not far inboard of the base. Thus the pattern skew is only a little less than the physical skew angle.

While the horizontal polarization is increased with element tilt the impact is modest for low tilt angles. As discussed above this can be bring an operating advantage for some and an annoyance for others. Extreme skew is not recommended. It is possible to compensate by raising the catenary height and tying the tops of the wire elements to the catenary with ropes. But if the catenary must be lower the tilt and horizontally polarization radiation are increased and the elements will likely need to be loaded in some fashion.

The direction of the element base offset only slightly affects performance of a driven array since the elements are far enough apart that the mutual impedance is low. However, element phasing must be changed to accommodate the geometry. That is why keeping the element bases at the ideal separation for an end-fire by skewing them on a virtual circle is not critical. Yagis are different and the adjustment for large skews will require model experimentation.

Element tilt and spacing has an impact on the feed point impedance. Modelling is helpful but you should measure the impedance after building one of these arrays. Design the matching network for the measured impedance.

Ideas, not designs

Details for the models in this article are deliberately avoided. I have not delved deep into these skewed vertical arrays and I am reluctant to provide detail that might be read as recommendations. The ideas presented in this article are just that: ideas. Should you wish to further explore these antennas, I suggest that you model alternatives and choose ones that suits your operating objectives and circumstances.

A few ideas for more interesting skew verticals that come to mind include:

• Pair of skewed end-fire arrays for 4 switchable directions
• 4-square with skewed vertical wires
• 3-element skewed vertical yagi, with 5 elements for 4 switchable directions
  

I have no intention of building a skewed vertical array so I have no real life experience to share. I can achieve my most urgent needs on 160 meters by shunt-feeding and driving both of my big towers.